Mr Renzi, the centre-left mayor of Florence, is meeting party leaders in an effort to form a new government.

Mr Grillo's anti-establishment Five Star Movement took a quarter of the vote in elections a year ago.

He called Mr Renzi "a young old man" and said "you're not credible... we are your natural opponents".

Mr Grillo said "we are the conservatives: I want water to be in public hands; you want to privatise it".

He also accused Mr Renzi, leader of the Democratic Party (PD), of representing "the banks and the strong powers".

Renzi offers new path

In a video clip of the meeting, Mr Renzi tried repeatedly to speak but was unable to interrupt Mr Grillo's tirade.

Mr Renzi later tweeted: "I'm so sorry for those who voted 5 Stars. You deserve more, my friends. But I promise you that we will change Italy, even for you."

Mr Grillo and MPs for his Five Star Movement have refused to reach any coalition deals with the mainstream parties. They accuse Italy's traditional political elite of ruining the country.

At the meeting on Wednesday Mr Renzi told him he was "not seeking any old-style agreement, none at all".

"We're not asking you for votes of confidence or otherwise. We want to tell you what we aim to do in the next three to four months," he said.

Mr Renzi presents himself as a break with the past, but in order to get his reforms adopted he needs to win support from centrist MPs. He may not, however, need to rely on support from ex-Prime Minister Silvio Berlusconi's Forza Italia party.

Mr Renzi has ambitions for electoral reform, a slimmed-down bureaucracy and labour market reform to haul Italy out of its economic malaise.